Unveiling the role of a ground state charge transfer complex in carbon nanoparticles for highly efficient metal-free solar hydrogen production

Carbon-based nanoparticles (CNPs) are exciting metal-free photocatalysts for solar hydrogen production. However, significant challenges remain in elucidating their complex structures and identifying the potential catalytic sites. Herein, we report a specific ground-state charge-transfer complex between nitrogen, phosphorus-co-functionalized carbon-based nanoparticles (N, P-CNPs) and a sacrificial electron donor (SED) namely triethanolamine (TEOA), defined as N, P-CNPs (TEOA). Optical and surface analytical investigations show the initial snapshot of the ground-state charge-transfer complex, which is further clarified through femtosecond transient absorption spectroscopy, electrochemical study, and computational analysis. Results suggest the crucial role of phosphorus functionalization in establishing the ground-state charge-transfer complex. The long-lived free carriers in the N, P-CNP (TEOA) charge transfer state stimulate highly efficient photocatalytic solar hydrogen production rates of 1.02 mmol g-1 h-1 and 651 mu mol g-1 h-1 from normal (mili-Q) and natural sea water, respectively, without any metal co-catalyst. Notably, TEOA plays a dual role; it forms a unique ground-state charge-transfer state with N, P-CNPs, and the excess TEOA molecules act as a potential hole-scavenger to boost the photocatalytic hydrogen generation process. A ground state charge transfer complex of N, P-CNPs and TEOA promotes long-lived free carriers and modifies the electronic energy levels for complete metal-free solar H2 generation. Free TEOA molecules act as effective SEDs during photocatalysis.